One measure of the power and utility of integrated circuits is the functional density of electronic devices. The value of integrated circuits is frequently directly dependent upon that functional density. Consequently, substantial efforts are expended in search of ways to downscale the size of electronic devices. In recent years, the development of "nanoelectronics" has promised substantial reductions in device geometries.
In practice, however, nanoelectronics (which make use of the quantum effects of charge carriers) have not been widely implemented for the very reason that they may be so valuable: their size. Conventional fabrication techniques have simply been unable to produce practically usable integrated circuits implementing quantum effect devices. The necessarily small size of quantum effect devices makes their fabrication under conventional techniques difficult. Another significant limitation associated with quantum effect devices is the frequent requirement that they be operated at low temperatures.
Very recently, however, quantum effect devices that overcome many of these barriers to practical use have been disclosed. For example, Yang, Kao, and Shih have disclosed a Stark-effect transistor in their paper "New Field-Effect Resonant Tunneling Transistor: Observation Of Oscillatory Transconductance," Appl. Phys. Lett., 55(26), Dec. 25, 1989, pp. 2742-2744. As another example, room temperature operation of resonant-tunneling hot electron transistors has been disclosed in "Room Temperature Hot Electron Transistors With InAs-Notched Resonant-Tunneling-Diode Injector," Japanese Journal of Applied Physics. Vol. 30, No. 5, May 1991, pp. 921-925 by Seabaugh, Kao, Randall, Frensley, and Khatibzadeh.
While these recent developments in nanoelectronics have allowed for practically usable devices, no structure has yet been disclosed that allows for different kinds of quantum effect devices to be fabricated in the same epitaxial stack. This integration is desirable as each transistor has its own unique advantage: the horizontal transistor, for its low power consumption, multiple negative transconductance characteristic, and high functional density, and the vertical transistor for its high current density and high speed of operation. Therefore, a need has arisen for an epitaxial stack that allows for integration of different types of nanoelectronic quantum effect devices.